1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices, and more particularly, to a method for manufacturing semiconductor devices having an epitaxial source/drain.
2. Description of the Prior Art
Epitaxial structures are prevalently used in a wide variety of semiconductor applications. For example, the prior art usually forms epitaxial structures in a single crystal substrate by performing a selective epitaxial growth (hereinafter abbreviated as SEG) method. Since the epitaxial structures have the crystalline orientation almost identical to the crystalline orientation of the substrate, the epitaxial structures serves as a raised source/drain or a recessed source/drain for the semiconductor device. Because the lattice constant of the epitaxial structures is larger than that of the silicon substrate, a strain stress is generated to the channel region of the meta-oxide semiconductor (hereinafter abbreviated as MOS) transistor device. Accordingly, carrier mobility in the channel region is improved and the speed of the MOS transistor device is increased. In other words, the epitaxial structures serve as a selective strain scheme (SSS) by rendering strain stress to improve the carrier mobility.
Although the epitaxial structures efficiently improve device performance, it increases complexity of the semiconductor fabrication and difficulties of process control. For example, masks are required to block regions where the epitaxial structures are not to be formed. It is found the epitaxial structures are damaged or consumed, and the strain stress is consequently reduced when removing the masks. That is, selective strain scheme (SSS) suffers strain stress loss due to mask removal and thus solution is in need.